1. Field of the Invention
The present invention relates to the control of switched reluctance machines, particularly those machines which are operated without a sensor to measure rotor position.
2. Description of Related Art
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This type is generally referred to as a synchronous reluctance machine, and it may be operated as a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This second type of reluctance machine is generally known as a switched reluctance machine and it may also be operated as a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, xe2x80x9cThe characteristics, design and application of switched reluctance motors and drivesxe2x80x9d by Stephenson and Blake, PCIM""93, Nxc3xcrnberg, Jun. 21-24, 1993, incorporated herein by reference. That paper describes in some detail the features of the switched reluctance machine which together produce the characteristic cyclically varying inductance of the phase windings.
FIG. 1 shows the principal components of a typical switched reluctance drive system. The input DC power supply 11 can be either a battery or a rectified and filtered AC supply and can be fixed or variable in magnitude. In some known drives, the power supply 11 includes a resonant circuit which produces a DC voltage which rapidly varies between zero and a predetermined value to allow zero voltage switching of the power switches. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals indicating the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms, for example it may take the form of hardware, as shown schematically in FIG. 1. In other systems, the position detector can be a software algorithm which calculates or estimates the position from other monitored parameters of the drive system. These systems are often called xe2x80x9csensorless position detector systemsxe2x80x9d since they do not use a physical transducer associated with the rotor which measures the position. As is well known in the art, many different approaches have been adopted in the quest for a reliable sensorless system. Some of these approaches are discussed below.
The energization of the phase windings in a switched reluctance machine depends on detection of the angular position of the rotor. This may be explained by reference to FIGS. 2 and 3, which illustrate the switching of a reluctance machine operating as a motor. FIG. 2 generally shows a rotor 24 with a rotor pole 20 approaching a stator pole 21 of a stator 25 according to arrow 22. As illustrated in FIGS. 2 and 3, a portion 23 of a complete phase winding 16 is wound around the stator pole 21. When the portion 23 of the phase winding 16 around stator pole 21 is energized, a force will be exerted on the rotor, tending to pull rotor pole 20 into alignment with stator pole 21. FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16, including the portion 23 around stator pole 21. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of lamination geometry, winding topology and switching circuitry are known in the art: some of these are discussed in the Stephenson and Blake paper cited above. When the phase winding of a switched reluctance machine is energized in the manner described above, the magnetic field set up by the flux in the magnetic circuit gives rise to the circumferential forces which, as described, act to pull the rotor poles into line with the stator poles.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the xe2x80x9cturn-on anglexe2x80x9d, xcex8ON), the controller 14 provides switching signals to turn on both switching devices 31 and 32. When the switching devices 31 and 32 are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air 10 gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches 31 and 32 and the phase winding 23. Current feedback is generally employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices 31 and/or 32 on and off. FIG. 4(a) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle xcex8ON is often chosen to be the rotor position where the center-line of an inter-polar space on the rotor is aligned with the center-line of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the xe2x80x9cfreewheeling anglexe2x80x9d, xcex8FW. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in FIG. 2) one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the diodes 33/34 (in this example 34). During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the xe2x80x9cturn-off anglexe2x80x9d, xcex8OFF, (e.g. when the center-line of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches 31 and 32 are turned off and the current in phase winding 23 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease. It is known in the art to use other switching angles and other current control regimes.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a xe2x80x9csingle-pulsexe2x80x9d mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. FIG. 4(b) shows a typical such single-pulse current waveform where the freewheel angle is zero. It is well known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
Many sensorless position detection systems are reviewed and categorized in xe2x80x9cSensorless methods for determining the rotor position of switched reluctance motorsxe2x80x9d, Ray et al, Proc EPE""93 Conference, Brighton, UK, Sep. 13-16, 1993, Vol. 6, pp. 7-13, which is incorporated herein by reference. The authors describe methods which are suitable for operation in the chopping mode or in the single-pulse mode. Most of the methods which are suitable for operation in the chopping mode use diagnostic pulses of some sort which are injected into an idle phase winding. By monitoring the result of these pulses, the control system is able to estimate the rotor position and hence determine when the main excitation should be applied to and removed from the phase windings.
One well-known implementation of this approach is described in xe2x80x9cA new sensorless position detector for SR drivesxe2x80x9d by Mvungi et al, Proc PEVD Conf, IEE Pub""n No 324, London, Jul. 17-19, 1990, pp. 249-252, which is incorporated herein by reference. This uses a region of the rotor cycle when the phase would otherwise be idle. Pulses are injected at a frequency which is high compared to the rotor speedxe2x80x94the paper suggests a frequency of 3.3 kHz is possible for the power switches being used. This appears to be a trade-off between loss in the switches and the desire to obtain as much position information as is possible during the window of opportunity when the phase is not required to contribute to torque production.
However it has been found with such systems that the injection of the pulses into the machine causes acoustic noise at the frequency of injection due to the distortion of the stator in the presence of the forces created by the pulse. The frequencies required are typically in the range 1-4kHz, a region where the human ear is particularly sensitive. The net result is that in such sensorless systems, there may be an audible buzz at the pulse injection frequency which may be found objectionable.
It is an object of this invention to provide a sensorless position detection method for a switched reluctance drive which has reduced noise emission. Embodiments of the present invention are generally applicable to switched reluctance machines operating as motors or generators.
According to an embodiment of the invention there is provided a switched reluctance drive comprising a reluctance machine having a rotor, a stator and at least one phase winding, a controller for controlling the output of the machine and means for injecting diagnostic pulses into at least one of the phase windings for rotor position detection, the controller being operable to determine the position of the rotor relative to the stator according to an effect of the diagnostic pulses and to actuate the means for injecting according to a variable frequency to reduce the perceived noise emitted from the drive.
Embodiments of the invention allow the buzz associated with the diagnostic pulses to be concealed within other events associated with the switching of the reluctance machine. The energy actually associated with the pulses may not be reduced, but the perception is that they have been removed. Thus, the operation of the machine becomes less objectionable to the human ear.
Embodiments of the invention may be arranged to vary the frequency in relation to rotor speed or it may be randomly variable. Pseudo-random frequency hopping could be used.
The pulses are preferably injected in an idle period of the or each phase. The pulses are also preferably voltage pulses.
To take account of the need to operate the machine at low speeds, a lower limit is preferably placed on the rotor speed at which the pulse frequency is made variable in accordance with rotor speed. Similarly, it is also desirable to impose an upper threshold on the variability of the pulse frequency with rotor speed. Furthermore, the transition between constant frequency and frequency variable with rotor speed may be made gradual to avoid any abrupt change between the two.
Embodiments of the invention also extend to a method of reducing the noise emitted from a switched reluctance machine comprising: controlling the output of the machine by the energization of the phase windings according to the position of the rotor relative to the stator; injecting diagnostic pulses into at least one of the windings; determining the rotor position from the effect of the diagnostic pulses; and varying the frequency of the pulses to reduce the perceived noise emitted from the machine.